1. Field of the Invention
This invention relates to a monocrystalline waveguide material for use, for example, in an SHG (second harmonic generation) source for light recording and measurement and, more particularly, relates to a lithium tantalate monocrystal with improved light transmittance by the addition of Mg, a monocrystalline material for use in an optical waveguide, a monocrystal substrate, and a photo element incorporating this.
2. Related Art
Generally, in fields such as optical recording, a shortening in the wavelength of laser sources is strongly desired for improvement of recording density. The shorter the wavelength, the higher the recording density. Currently, laser sources in the range of 280-400 nm of wavelength, or blue to near ultraviolet are in demand. To realize such a source, the conversion of a semiconductor laser with a wavelength of 830 nm into a blue light of half that wavelength, 415 nm, by use of optical waveguide SHG elements has gained considerable attention. As discussed on pages 731-732 of the Electronics Letters, 25, 11 (1989), for example, a method of performing phase matching by using periodic polarization Inversion has been proposed.
Concerning a suitable monocrystalline waveguide material for use in such an SHG element, a lithium niobate (hereinafter abbreviated as LN) monocrystal has been generally used up to now, having a large non-linear optical coefficient (refer to U.S. Pat. Nos. 3,262,058 and 3,665,205). To produce this SHG element, periodic grids are produced on an LiNbO.sub.3 substrate by Ti diffusion and heated up to about 1100.degree. C. to selectively invert the polarization of the periodic grid layers, as shown in FIG. 4, for example. An optical waveguide is then produced by means of the proton exchange method wherein a fundamental wave is injected to obtain SHG light. However, LN monocrystals cannot be used in the short wavelength range of up to 280 nm currently required, because the fundamental absorption edge, which indicates wavelength below which the light transmittance of the crystal becomes zero, is in the vicinity of 320 nm.
In contrast, ever since it was reported that a low loss optical waveguide could be produced by the proton exchange method as discussed on pages 45-60 of Nikkei New Materials (Nov. 4, 1991), lithium tantalate (hereinafter abbreviated as LT) monocrystal has gained considerable attention in recent years as a substrate material for a wavelength conversion element generating second harmonic wave by the quasi-phase matching method for the reason that lithium tantalate crystal exhibits excellent photorefractive damage resistance.
This LT monocrystal has an ilmenite structure as does LN monocrystal and is a ferroelectric crystal with a melting point of about 1650.degree. C. and Curie temperature of about 600.degree. C. It is usually grown from melt by the Czochralski method using an iridium crucible in an inert atmosphere with or without oxygen. Because the resultant monocrystal is in multi-domain state, a poling process is performed by slow cooling with an applied electric field in the air or an oxygen atmosphere after maintaining the crystal above the Curie temperature. Later, the crystal is processed into a wafer and used in large quantities as substrates for surface acoustic wave elements. Although the crystal is usually of brown or light yellow color due to the mixture of precious and transient metals from the crucible and of impurities from the starting materials and refractories in the furnace, it is used as substrates for surface acoustic wave elements without any associated problems. However, this LT monocrystal has not been used very much for optical applications up to now because of its small birefringence. To utilize LT as SHG material, the method has been attempted as discussed on pages 2732-2734 of Apply. Phys. Lett. 58 (24) (1991), for example, wherein periodic grids are produced by the proton exchange method instead of Ti diffusion, heated up to about 600.degree. C. to selectively invert the polarization of the periodic grid layers, thereby producing an optical waveguide by the proton exchange method. This is illustrated in FIG. 5. Such a photo device generates about 15 mW SHG light with a waveguide width of about 4 .mu.m a waveguide depth of about 2 .mu.m and a wavelength of about 433 nm, resulting in & light power density as large as about 188 kW/cm.sup.2.
Other known inventions relating to LT monocrystal include, for example, that relating to an LT optical waveguide produced on an LT substrate with 10 mol % or less of MgO added, as presented in Japanese Patent Publication (Koukoku) No. 32843/1985. This invention alms at reducing the refractive index by the addition of MgO to improve the relative refractive index of the optical waveguide to confine light.
In addition, potassium titanium phosphate (hereinafter abbreviated as KTP) monocrystal, which has the fundamental absorption edge in the vicinity of 350 nm, is widely used for the generation of green light having a slightly longer wavelength (U.S. Pat. No. 5,039,187)
However, because lithium tantalate monocrystal produced by the aforementioned prior art is of brown or light yellow color due to the mixture of precious and transient metals from the crucible and of impurities from the starting materials and refractories in the furnace and absorbs light in the visible ray range, it cannot be used for photo elements using visible rays. Also, impurities increase photorefractive damage, presenting a big problem by hindering the practical optical use of the monocrystal.
The main cause of the aforementioned light absorption is assumed to be transient metal impurities such as iron contained in the crystal. If the impurities are reduced to 1 ppm or less, for example, light transmittance is obviously improved. However, it is difficult to eliminate impurities completely. The reasons are that in the growth of oxide monocrystals, the purity of available materials is about 4N-5N and there is also a certain limit to the reduction in impurities since growing crystals imbibe impurities from the crucible material and refractory insulating materials in the furnace making it difficult to produce the crystals to the purity comparable to that of a semiconductor. In addition, oxygen vacancies are also taken into the crystal depending on the growth and the heat treatment conditions, to also cause coloring and light absorption.
The compact, light-weight blue light sources developed in recent years can only generate SHG light with a minimum wavelength of about 400 nm. Reasons for this include the fact that light absorption by the lithium tantalate monocrystal substrate is large in the visible ray range and that the fundamental absorption edge is around 280 nm or longer.
Furthermore, the Inventor of the present invention has found that LT monocrystal has a serious problem in that while it has high feasibility in the short wavelength range because the fundamental absorption edge is in the vicinity of 270 nm, the light transmittance drastically decreases in the 280-400 nm band.
Although light induced refractive index change called photorefractive damage present a problem for a monocrystal used in an optical use, especially in an optical waveguide where light energy density is high, few inventions for improvement have been made because the monocrystal has so far been limited in its application to SAW.
Although it has been said that lithium tantalate monocrystal produced by the aforementioned prior art is higher in photorefractive damage resistance than lithium niobate monocrystal, the inventor of the present invention et al have found it to be more sensitive to photorefractive damage than lithium niobate monocrystal and that photorefractive damage occurs in the practical optical use of wavelength conversion elements, hindering the practical use of the monocrystal. Photorefractive damage mentioned herein means local variation in the refractive index of crystal due to injected laser light, known as light induced refractive index change. The cause of this photorefractive damage is said to be transient metal impurities contained in the crystal and this phenomenon is specifically explained by the change in the valence state of Fe ions within the crystal, as follows. When light is injected in a direction not parallel to the Z (optical) axis of the crystal, Fe.sup.2+ ions in the high light intensity part of the light irradiated zone are excited and emit electrons in the conduction band, thereby changing to Fe.sup.3+ ions. The electrons thus generated are generally captured by other Fe.sup.3+ ions present in the non-irradiated or low irradiated zones of the crystal and these ions are changed to Fe.sup.2+ ions. The overall effect of such a phenomenon results in changes in space charge distribution, and in consequence, in local refractive index non-uniformity through the electro-optical effect of the crystal itself. When crystal is used as substrates for optical applications such as light modulators and wavelength conversion elements, considerable problems arise due to refractive index changes in the light irradiated zone preventing the element from operating stably and inhibiting the full use of the properties inherent to the crystal.
The shorter the light wavelength, the more increased the photorefractive damage Photorefractive damage is therefore more serious problem to elements using shorter wavelength light. It has been said that the occurrence of photorefractive damage is especially remarkable in lithium niobate monocrystal and that lithium tantalate is high in photorefractive damage resistance as compared with lithium niobate monocrystal. Lithium tantalate monocrystal has so far been used for surface acoustic elements, and in this application, although the crystal contains large amounts of sub-grain boundaries and transient metal impurities, they do not have any substantially negative effect on the element properties. In optical applications, however, problems occur such as the unstable operation of elements due to photorefractive damage and light scattering at sub-grain boundaries. Also, even with the electric field annealing method, which has so far proven to be effective for improving photorefractive damage resistance, photorefractive damage occurs as the power density of the element increases. For these reasons, for elements using short wavelength light, it has not been possible to obtain crystal having satisfactory photorefractive damage resistance.
When lithium niobate monocrystal is used for substrates, it suffers from the effect of fundamental absorption edge at a wavelength of 400 nm because the fundamental absorption edge of lithium niobate monocrystal is in the vicinity of 320 nm, resulting in poor light transmittance and it is therefore difficult to generate shorter wavelength light than in the case of lithium tantalate. In order to develop a shorter wavelength light source, a monocrystalline substrate material having excellent light transmittance In a short wavelength range is required. However, no suitable monocrystalline waveguide material that can be used in the short wavelength range has been available because of problems such as light absorption due to impurities contained in conventional lithium tantalate monocrystal and due to coloring by oxygen vacancies.
In addition, KTP monocrystal cannot be used in a shorter wavelength range than LN monocrystal because the fundamental absorption edge is in the vicinity of 350 nm.